Electromagnetic sound reproducer



Nov. 3, 1931. B. F. Mlr-:ssNER 11,830,401

l ELECTROMAGNETIC SOUND REPRODUCER l Filed March '7. 1929 am/WM? @w www2W spea C0 tofore realized.

Patented Nov. 3, 1931 UNITED STATES PATENT ori-ICE BENJAMIN r. umssNni,or excam HILLS, Naw JERSEY; vaasIeNoiz. 'ro mmssima 1N- VENTIONS INC.,OF SHORT HILLS, yNEW JERSEY, CORPORATION 0F NEW mmc'rnoncNnTIc soUNDnnrnonucnn i appnmon mea laren 1, 1929. serial No. ''ilagsii;

Theipresent invention relates 'to electro-l magnetic sound reproducers,and more particularly to those reproducers including provision V'formaintaining therein an intense `5 magnetic field.

The invention contemplates in particular the so-called Ielectrodyna'mictype of sound reproducer in which an intense magnetic field ismaintained for .interaction with a dynamic coil excited with the soundrepresenting currents.

An object of the invention is the r'ovision of an arrangement whereb theeld producing coil of such a repro ucer can be excited with auni-directional pulsating current without disturbing effects on thedynamic coil, and in particular excited with incompletely filteredrectified alternating current, as is had in the now usual practiceofenergizing the three-electrode vacuum tubes of amplifying systems fromcommercial alternating current sources.

In the usual mode of construction of electrodiynamic reproducers asso-called loud ers for radio broadcast receivers and electrical`phonographs the field coil is made of such electricalV dimensions thatits inductive impedance is of suitable value as to be well adapted tofunction as the high inductive impedance in a filter system associatedwith an alternating current rectifier for su plying the plate and gridenergy for amp ifier tubes.

ployed, of such filter systems, thereby saving weight, space and cost ofmaterial. However, the incompletelyfiltered current at'the Vpoint in afilter system where the field coil must be used in orderto replace achoke coil A l undesirably intense. l

Endeavor has heretofore been made to neutralize thel effect of thefluctuating field on the dynamic coil of the speaker-with but littlesuccess, and one feature of my invention is the provision of means forneutralizin the hum'effect to a degree of perfection not erey Such useeliminates the usual choke coil, or one coil if two or more are em- Myinvention will be readily understood from the following description inconnection with the figures of the accompanying drawings in which likereference characters represent like parts in the several figures.

Fig. 1 shows in a cross sectional view the usual form-of construction ofso-called dynamic speakers arranged to include features of my presentinvention. v

Fig. 2 diagrammatically illustrates suitable electrical connections fortaking advantage of the features of Fi 1.

Fig. 3 shows a fragment o the device of Fig. 1 sufiicient to illustratea modified form of my invention.

Fig. 4 shows in detail a useful manner of mounting a dynamic coil andattached reproducing element.

Referring to Fig. 1, a magnetic circuit of the usual form ofelectrodynamic reproducer is shown to includea cylindrical element 1 ofcast iron or other magnetic material having a closure at one end by anintegral part of the casing, and a closure at the other end comprisl inga disk 4 of soft iron or other suitable magnetic material secured to thewall of the cylin- .der 1 by means of a suitable series of screws 9. Aceni ral corel 2 of soft iron or other suitable magnetic material isbolted to the base of the c lindrical element 1 by a bolt 3' as'shown,tliecore 2 extending throu h a circular opening in the front disc 4 as sown, the opening in the disc 4 being of sli htly larger diameter thanthe diameter of t e core 2 in order to accommodate a coil 6 secured to acylindrical projection 7 of a conical sound producing element 7. f

`A coil 5 usually comprising many turns of fine wire encircles the core2, and is arranged with in ut terminals 13 and 14 through which't efelddproduciiig or energizing current is supplie With the arrangementshown, the energizing of the coil 5 with a unidirectional curcylindricalelement 1, and thence'through the 10c the gap portions. so that Whensound representing currents are applied to this coil through theterminals 15 and 16, there is a motion producing reaction set upbetweenthe coil and the intense magnetic field of the gap, `resulting insound reproducing motion being imparted to the cone 7 or otherassociated sound reproducing element. The usual construction ofelectrodynamic reproducer endeavors to make the air gap as short as thestructure of the coil 6 and its mounting on the cylindrical element 7will permit while leaving sufficient clearance to prevent frictionalcontacts, this in order that the flux in the gap may be maintained asintense as possible for a given structure ofmagnetic system and fieldproducing coil.

If the coil 5 is energized fromA a source. such as rectified andfiltered alternating current, having a pulsating component, obviouslythe resulting flux through the gap will be of unidirectional pulsatingcharacter. and Will react With the coil 6, with or Without the signalrepresenting current fioul. to proradio broadcast reception.

duce motion of the cone 7 that will cause a hum characteristic ofthelpulsations in the field exciting current, an undesirable feature insound reproducing devices in gencral` particularly those used forentertainment purposes such as phonographic reproduction or Because ofconvenience, the/distaste of the public for batteries and thecharacteristics of the electrical energy required. it is almostcompulsory to employ energy derived from .commercial alternating currentsources for energizing the field coil of current designs ofelectrodynamic reproducers. Further, the high inductanee available inthe field coil of commercial electrodynamic reproducers makes thesecoils particularly suitable as the inductive impedance element of thefilter systems commonly usedy in 'connection with filtering rectifiedalternating current for energizing the plate and grid electrodes ofthree-elecl trode vacuum tube amplifiers used in radio receivers and2electrical phonograph reproducers, and such use saves the expense,Weight and space of additional coils for filter purposes in addition toavoiding the employment of a. separate source of' current for energizingthe field coil.V For these reasons a satisfactralization.

tory arrangement vfor preventing the production of hum when energizingthe field coil with uni-directional pulsating current is of greatimportance, and the features ofmy present invention permit of this humelimination to most effective degree. y

It has heretofore beenv proposed to provide means, commonly known as abucking coil, in an effort to neutralize the effect of the fluctuatingfield on the dynamic or movable coil and I show a coil 13 in Fig. 1located adjacent the field coil 5 as illustrating the usual practicetowards this end. In this practice the coil 13 is connected in serieswith the dynamic coil 6, and of such polarity that the induction by thefluctuating field opposes the induction by the fluctuating field in coil6, the connection being made through terminals 19 and 20.

Experience has shown however that such a bucking coil has given verylittle improvement in the matter of hum production, it

being usualto find the hum when the coil isused about 1/3 of theintensity of the hum u'ithout the coil, but still sufficiently intenseto be unsatisfactory in the usual entertainment providing apparatus. v

On mere casual consideration it would seem reasonable to assume that abucking coil 13 could be so proportioned, or have its output socontrolled, as to mjake it possible toarrive at a proper degree of'neutralizing effect to satisfactorily opposethe hum producing effect inthe dynamic coil 6, but the fact is that no manner of skilledproportioning of the bucking coil 13, or its output, has heretoforesucceeded in arriving at the desired hum elimination results, Whereaswith the features of my present invention I am enabled to so far reducethe hum that for all prac-4 tical effect on human audition it may besaid to be Wholly eliminated.

In arriving at this result, I have found that the practice of locatingthe bucking coil with respect to the magnetic system in the usual Way asshown by the coil 13 in Fig.- 1

apparently permits of some induction in the coil by components ofmagnetic flux not linking -With the dynamic coil 6, probably resultingin a disturbancev of the induction inV the bucking coil to give adifferent Wave form therein from that in the dynamic coil, so that themere poling of the coil 13 to be generally opposed to the coil. 6 doesnot satisfy all of the requirements for complete neu- As a result ofthis appreciation of a diHerence in the induction effects I have soughtandfound a location of the bucking coil with respect to the dynamic coiland its relation to the magnetic flux of the system that markedimprovement results, and have .further foundl that by including ashielding effect in a Way to govern the magnetic induction in thebucking coil, I can control the production of a bucking induction insuch a Way as to ,bring about most complete neutraliz'ation. In Fig..1,I show 'my location of a bucking coil 8, of so-called pancake form, invery close relation to the gap in which the dynamic coil 6 is located,the bucking coil being shown to encircle the opening in the disc 4, onits inner side, and as lclose to the disc surface as practicalconsiderations. permit. With this location I found in experimenting witha commercial form of reproducer that the hum could be reduced to theorder of ,1f of that had without the use of a bucking coil, as comparedwith the reduction to 1/3 for the location previously discussed., Ifurther find that by covering the coil 8 with a shield of magneticmaterial 10, magnetically'secured to` the disc 4 by the screws 12 forinstance, that I was enabled to reduce' the hum to the order of 1/500thof that hadwithout any hum' vbucking co'l,

andcompared with the 1A, had with coil 3,

and the 1112 hadl with the coil 8 without the shielding, so much so thatthe hum is almost .humanly inaudible and the electrical energy of italmost impossible to measure with available measuring apparatus.

Heretofore it has been usual to employ a disc somewhat of .the structureof that of the disc 10 secured tothe inner side of the disc 4, having acentral hole therethrough of substantially the same diameter as thediameter of the core 2 in order'to provide means for accuratelycentering the core 2 in the opening of the disc 4 to provide a gap ofpermanent dimensions for the dynamic coil 6. In order vto prevent thissupporting and aligning disc from short-circuitng the inagnetic fluxfrom the coil 6, it yhas been usual 'to make this disc of non-magneticmaterial such as brass. In order to use the disc as an effectivemagnetic shield for my bucking coil 8, I have made the outer portion ofit of preferably the best grade of magnetic material, such as pure softiron,-but in order to still utilizel the useful function of the disccentering the core 2, I have made the portion of the opening thereinwhich makes contact with the 'core 2 of non-magnetic material, as brass,having found that a suitable radial depth of Anon-magnetic material canbe satisfactorily pressed into a larger opening of themagnetic portionofthe disc. As an eXample'I employ a depth of nonmag netic-material of1/8 inch, having found that with the external diameter of thenon-magnetic portion of the disc substantially equal to, and preferablyless than, the internal diameter of the'bucking coil 8 entirelysatisfactory results are had.

While I have referred to the shield 10-as magnetically secured to thedisc4, this is merely a'preferred arrangement. ,The shield 10 can bemoved anywhere along the core 2 between the coil 5 and discl 4, and thecoil 8l moved-to follow, and still obtain the beneficial results of myinvention, though not to the same degree. Also a construction in whichthe distance between field coil 5 and bucking coil 8 is increased, as-by deepening the cylindrical magnetic element will result in obtainingthe beneficial effects of my invention.

I havey further found that with a given structure the leffectiveness ofhum neutralizat1on can be affected by some suitable means for variablyextending orlessening the effective magnetic projection of the core 2 inorder to modify the magnetic fieldl components, as by a screw 12ofmagnetic material o erating in a threaded hole 12 in the' end of, Forexample, having a coil 8 and shield 10 that does not completelyneutralize for coil 6, improvement can be had by screwing element 12 inor out as found by trial lto obtain a further reduction in hum.- Theeffectiveness to the gap, so that a screw device 12 may core 2. I

prove of value as a final adjustment in quantity production ofreproducers to adjust for production differences in positioning coil 6.

An'adjusting device, such as the screw 12,

may be used to advantage in other ways. In

those uses of the reproducer with a. vacuum' tube amplifier systemenergized from the same source as the coil 5, it may be founduneconomical to try to eliminate all of the hum current arising fromsuch energizing, so that there is some residual 'hum currentaccompanying the sound representing currents ntroduced from the outputof the am lifier -iuto coil 6, which may be neutralized y the adjustmentof coil 8.

In Fig. 2 the connections for the coils are shown, the arrangementincluding another way of controlling the neutralizing as be tween thedynamic coil and the bucking coil. The core 2 of Fig. 1 is again shownwith field coil l5 connected to be ener ized by way of terminals 13 and14, as from a source 26 of periodically fluctuating unidirectionalcurrent. Dynamic coil 6 is shown connected to be supplied with soundrepresenting currents through a transformer 23 connected to, for

example, theoutput terminals 21 and 22 of an amphfer system. The coil 6is connected in-series with coil 8 through a potentiometer 24 across theterminals 17 and 18, or other suitable means, permitting adjusting themagnitudes of the hum bucking current collected in coil 8 applied tocoil 6, a control equivalent to that provided by the screw 12 in Figs. 1and 2, or which may be used in conjunction with an adjustment had by useof screw 12.

As a practical procedure the coil 8 maybe constructed with more turnsthan needed for .neutralizing for a normal position of coil 6,

and the potentiometer used for securing an adjustment for taking care ofproduction discrepancies in location of coil 6. )r the potentiometeradjustment may be utilized for correcting for residual hum currentcoming from a preceding amplifier. v

In the fragmentary Fig. 3 I show another manner of locating the buckingcoil 8 with respect to dynamic coil 6. In this arrangement the core 2 isstopped short of the opening in disc 4 so that a screw attachment 12,

with enlarged head as shown, may form` the annular gap with the openingin disc 4, with coil 6 loca ed in this gap as before. The screwattachment 12 has its shank threaded for adj ustably screwing into a4threaded hole` 12 in core 2. The coil 8 is inset under an overhang atthe rear of the head of screw attachment 12 as shown. This arrangementvtherefore provides for locatingv the 'bucking coil with respect to thedynamic coil so as air gap longitudinally as well as laterally.

In some dynam-ic speaker .construction provision is made for positioningthe coil 6 laterally in the gap by inserting a fiexible 'diaphragm 27 atthe truncated portion of the conical element 7 which diaphragm has acentral hole which permits of securing the center of the diaphragm to afixed point in the end ofcore 2, as by a bolt or screw, thus assuringthe cylindricalcoil support 7 always taking correct position in the gapwithI respect to core 2. The fiexible construction of the diaphragm 27is relied upon to permit coil 6 to move cone 7 in spite of the centralpinning of the diaphragm.

As an additional feature I employ a washer 28 of' other suitablearrangement'of material, such as rubber or cork, between the core 2 anddiaphragm 29, and which can thus be compressed within limits by bolt 29to permit of longitudinally setting the position of coil 6 in the gapfor final hum elimination adjustment in quantity production, and likeconditions making such adjustment desirable.

While I do" not wish to be understood as offering any specific theory insupport of the extraordinary results in hum elimination obtained -by mefrom relocating the bucking coil and\shielding it, yet it is thoughtthat a fuller mnderstanding of the principles involved will be had bythose skilled in the art by reference to practical observations made byme..

one-third of thehum had without the use of any coil. Upon moving thebucking coil to the position at the back of disc I found that I had toincrease the number of turns to secure the minimum hum, but reduced thehum to about one-twelfth of that had without any coil. Then with theshield 10 covering a bucking coil of turns I secured a reduction of thehum to about the 1/500th part of the hum without any coil, andsubstantially down to the limit of human audition.

The feature of having to increase the number of turns to secure minimumhum as the Abucking coil was moved away from the field lexisting alongthat part of the core 2 between the field coil and the gap,-that is,thatl portion of the total flux of the system not passing through thedynamic coil 6,.-and since the elimination of linkage of the buckingcoil with this stray field improved the neutralizing had it appearsreasonable that the poor neutralizing had with the bucking coil locatedclose to the field coil and within the stray field is in some wayassociated with the stray field component of the total flux which strayfield, because of encountering a different' magnetic reluctance, differsin phase from the field passing through the dynamic coil 6. Poorneutralizing can well be accounted for by a distortion differencebetween the induction in the buckingcoil and the dynamic coil makingexact opposing impossible, so that alnplitude adjustment is notaltogether effective.

It seems sound to reason that the stray field component, due todifferent'reluctance by reason of a longer air gap, is out of timeharmony or phase with the main field component continuing in the ironpath 0f core 2 to and through the principal but short gap,

so that the bucking coil having an induction A represented by the vectorsum of both'components is not in continuous opposed step to theinduction in 'the dynamic coil of the main field component only.

The additional and marked effectiveness of the shield 10 'lends weightto ,suspecting 'the trouble to rest with the different conduct of thestray field component. It is seen that without the shield it is stillpossible for stronglstray field from core 2 to the rear of the disc 4 tolink with the coil 8 without linking the dynamic coil, ,butwhich fieldis largely short circuited from coil 8 when the shield disc is in place.

In other words the fundamental require-` ment of reall effective humbuckingA is to have the buc g coil linked onl by flux undergoing thesame hysteretic in uences as 5 the iiux linking the dynamic coil.Because of the relation of the 100 turns in the dynamic coil to the 50turns in the bucking coil I have used, it is apparent that magnitude orvolt ,turns oer no diliiculty, being susceptibld to the usual principlesas long as like flux characteristics, vectorially speaking, are had -forboth coils. l

Theoretically, therefore, a perfect hum `bucking condition would beobtained if the two coils could be made equal and magneti- 1 callycoincident, but in view of the fact that both coils carry the soundrepresenting currents any approach to' this theoretically perfeetassociation for humc bucking must be avoided in order to avoidsubstantial sound current neutralization. By separating the coils asshown in Figs. 1 and 3 the mutual electrical couplin with respect tosoundl currents is reasonab y smalland of no practical consequence. j

It is obvious that the passage of sound current through the bucking coilamounts to a loss in eiliciency in proportion to the relative electricaland motional impedance of the bucking coil and electrical impedance ofthe dynamic lcoil, so that my ability to secure complete neutralizationof hum with a bucking coil having but one-half the turns of the dynamiccoil is of substantial value in maintaining e'ciency; l

. While I have disclosed my invention with particularity with respect tospecific embodiments thereof, no limitations by reason of such choicefor explanatory purposes are intended, the`invention being one of broadscope capable of numerous adaptations readily apparent to those skilledin the art.

Havingv fully. described my invention, I claim: Y

1. In an electromagnetic sound reproducer the combination of a magnetenergized by direct current, an air gap in the magnetic circuit of saidmagnet, a dynamic coil adapted to be energized with sound representingcurrents operatively located in said air gap, and means -for eliminatingdisturbances in vsaid dynamic coil due to periodic fluctuations of saiddirect current comprising a. second coil connected in opposing series,with saidv dy# namic coil, said second coil being so closely associatedwith said air gap that it is substantially independent of the magneticleak- .age of said system not eiective on said dynamic coil.

' '2. In an electromagnetic sound reproducer the combination 'of amagnet energized by direct current, an air gap in the magnetic circuitof said magneta dynamic coil adapted Y to be energized with soundrepresenting cur- 05 rent operatively locatedlin sald air gap, and

dynamic coil due to periodic pulsations of said direct currentcomprising a second coil connected in opposing series with said dynamiccoil, said second coil being magnetically shielded from at least aportion of the stray fields of said magnetic system extraneouis1 to saidgap not effective on said dynamic co 3. In an electromagnetic soundreproducer the combination of a magnet including means for theenergizing thereof by direct current, an air gap in the magnetic circuitof said magnet, a dynamic coil adapted to be energized with soundrepresenting current operatively located in said air gap, a second coilassociated with said magnetic system con- 'means for eliminatingdisturbances in said 'lliA nected in opposings'eries with said dynamiccoil, and a magnetic shield substantially iso- .lating said -second coilfrom stray fields of said magnetic system extraneous to said gap noteective on said dynamic coil.

4. In an electrodynamic sound reproducer thecombination of a magnetincluding means for the energizing thereof by/drect current lhaving analternating component, an airgap in the magnetic circuit of said magnet,a dynamic coil adapted to be energized with sound representing currentoperatively located in said air gap, a second coil associated with saidmagnetic system in close relation to said air gap, said coil beingconnected in opposing series with said dynamic coil, and magneticallyeifectivemeans for altering the stray field conditions lwith respect toone or both of said coils in the neighborhood of said gap so as toproduce in said two coils harmonious flux inter-linkages as induced fromsaid alternating component of thelield.I

5. In an electrodynamic sound reproducer the combination of a magnetincluding means for the energizing thereof by direct current,

`1 an annular airgap in the magnetic circuit of said magnet' formed by acircular magnetic core projecting into a circular openingin a magneticdisc, a dynamic coil operatively positioned in said airgap, a secondcoil encircling said circular opening on one side of said disc, saidsecond, coil being connected in opposing series with said dynamic coil,and a shield of magnetic material magnetically associated with said disccovering said second coil and through which said core extends inprojecting into said circular opening, the opening in said shieldthrough which s'ald core ex.ends being lined with non-magnetic material,whereby said shield serves the additional function of xedly positioningsaid core in the circular opening of said disc without undulyshort-circuiting magnetic flux from said annular airgap.

6. In an electromagnetic sound reproducer the combination of a magnetincluding means for the energizing thereof by direct current, an annularairgap in the magnetic circuit of said magnet formed by a circularmagnetic core projecting into a circular opening in a magnetic disc, adynamic coil operatively positioned in said airgap, a second coilencircling said circular opening on one side of saiddisc, said secondcoil being connected in opposing series with said dynamic coil, and ashield of magnetic material substantially covering said coil having anopening through which said core extends in projecting through saidcircular opening without said core making magnetic contact therewith.

7. In an electrodynamic sound reproducer the combination of a magnetincluding a disc of magnetic material having a circular opening therein,a circular core projecting through said opening of such diameter as toform an annular airgap with said disc, a direct current coil onsaid corefor the energizing of said magnet, a dynamic coil operatively positionedin said airgap, and means for eliminating disturbances in said dynamiccoil due to periodic fluctuations of said direct current comprising asecond coil encircling said core between said direct current coil andsaid airgap, said second coil being connected in opposing series withsaid dynamic coil, and a shield of magnetic material interposed betweensaid second 4coil and said direct current coil.

' 8. In an electromagnetic sound reproducer n the combination of amagnet energized by direct current, an airgap in the magnetic circuit ofsaid magnet5 a dynamic coil adapted 'i 35 to be energized with soundrepresenting current operatively positioned 1n said airgap, and meansfor eliminating' disturbances in said dynamic coil due to periodicuctuations of said direct current comprising a second 4u coil of less.turns than said dynamlc coil connected in opposing series therewith,Vsaid second` coil being associated with said magnetic circuit so as tohave a' greater flux linkage therefrom than is had by said dynamic coil,

and furtherso associated with said magnetic circuit as to besubstantially independent of stray flux extraneous to saidy aix-gap notlinkin said dynamic coil.

n witness whereof, I have hereunto subscribed my name this 5th day ofMarch, 1929.

` BENJAMIN F. MIESSNER. v

DISCLAIMER 1,830;v401.-Benjamin F. MiessnergfShort Hills, N. J.ELEc'rBoMAGNE'rIc SOUND REPRODUCER. Patent dated November 3, 1931.Disclaimer filed November 25, 1932, by the assignee, Me'essnerInventions, Inc. l

Hereby enters this disclaimer to 4that part of the claim in saidspecication which is in the following words, to wit:

* 3. In an electromagnetic sound reproducer'the combination of. a magnetincluding means for the energizing tli'ereof ,by direct current, an airgap in the magnetic circuit ofA said magnet,y a dynamicv colll adaptedto be energize with sound rpresenting current operatively located saidgap, a second coil associated with sai magnetic `system connected inopposing series with said dynamlc coll, and a magnetic shieldsubstantially isolating said second coil from stray fields of saidmagnetic system vextraneous to said gap not effective on said dyanmiccoil.

[Oficial Gazette December A20, 1982.]

